1. Field of the Invention
The present invention relates to a light emitting material to be used for a light emitting layer provided in an organic electroluminescent device, a method of manufacturing the light emitting material, and a method of forming the light emitting layer.
2. Description of the Prior Art
There is known an organic electroluminescent device (hereinafter, referred to as an “organic EL device”). The organic EL device has a structure in which at least one light emitting organic layer (organic electroluminescent layer) is provided between a cathode and an anode. Such an organic EL device can significantly reduce a voltage to be applied as compared with an inorganic EL device. Further, it is also possible to manufacture devices that can provide various luminescent colors.
Currently, in order to obtain higher-performance organic EL devices, various researches are being actively carried out in developments and improvements of materials to be used as well as device structures thereof.
Up to now, organic EL devices that can provide various luminescent colors or organic EL devices that have high luminance and high efficiency have been already developed. However, in order to realize their various practical uses such as application to a picture element of a display or a light source, further researches are being carried out.
However, from the viewpoint of practical use, the existing organic EL devices still have a problem in that light-emission luminance thereof is decreased or deteriorated when it is used over a long period of time, and therefore there is a demand for the establishment of technical measures to solve the problem.
As for a method for suppressing the decrease of light-emission luminance of an organic EL device, a method using a high-purity organic compound has been proposed (see Japanese Patent Laid-open No. 2002-175885, for example). Japanese Patent Laid-open No. 2002-175885 discloses an organic EL device, in which the content of a halogen-containing compound (impurities) contained in a material constituting the device is made less than 1,000 ppm, thereby suppressing the decrease of light-emission luminance which will occur when it is used over a long period of time.
However, specific indexes of the relationship between the decrease of light-emission luminance in an organic EL device and the kinds of impurities contained in the constituent materials to be used and their amounts contained therein have not yet been established. Thus, further researches are being carried out toward realizing the practical uses.